I. Field of the Invention
The present invention relates to a process for producing a cadmium sulfide layer on a cadmium telluride surface by exposing a surface of cadmium telluride to a hydrogen sulfide plasma.
II. Description of the Prior Art
Cadmium telluride shows promise for terrestrial photovoltaic applications primarily because of its direct band gap of 1.5 eV, which is optimum for solar energy conversion at air mass 1. Especially beneficial is a cadmium telluride film having relatively large cadmium telluride grain sizes, with such sizes preferably above about 20 .mu.m. This larger grain size reduces surface recombination and interface state density with increased efficiency being directly proportional to grain size. Conversely, films of a smaller grain size have numerous boundaries of individual crystals, and such boundaries critically inhibit the flow of charge carriers through the film.
Commonly-assigned copending U.S. patent application Ser. No. 08/308,362, filed Sep. 19, 2994, (now U.S. Pat. No. 5,484,736, issued Jan. 16, 1996) and incorporated herein by reference in its entirety, teaches a process for producing large grain cadmium telluride. In summary, the process taught by the copending application produces a cadmium telluride polycrystalline film having grain sizes greater than about 20 .mu.m by first depositing cadmium telluride on a substrate and subsequently exposing the deposited cadmium telluride to an inert gas such as helium at an elevated temperature. While a large-grain cadmium telluride surface significantly improves solar energy collecting efficiency in devices employing such a surface, untoward performance effects can still occur because of surface states, surface recombination velocity, and the like.
It is therefore apparent that passivation of the cadmium telluride surface, even though it may be comprised of large-grain cadmium telluride, would be highly beneficial to the efficiency of such solar-energy collecting devices. Accordingly, a primary object of the present invention is to provide a process for producing on a cadmium telluride surface a cadmium sulfide layer which functions to passivate surface states and thereby reduce surface recombination velocity.
Another object of the present invention is to provide a process for producing the cadmium sulfide layer wherein a plasma application of hydrogen sulfide to the cadmium telluride surface under appropriate conditions yields the layer and produces a heterojunction at the cadmium sulfide/cadmium telluride interface.
These and other objects of the present invention will become apparent throughout the description of the invention which now follows.